Molecular biologiczl techniques are being applied to evaluate yeast speciation and strain variability. DNA base composition, genome size, amount of repetitive DNA and degree of DNA homology will be determined for various yeasts. Medically, industrially, and ecologically important yeasts are included in this investigation. An emphasis on imperfect yeasts is made to provide necessary information on their proper indentification and relationship with perfect yeasts. This research project will provide valuable contributions to the understanding of the relatedness among yeasts and to the knowledge of evolutionary trends and developments. Thus far, 113 strains of yeasts have been examined for DNA base compostion. Many of these strains were exposed to modificatins of the usual isolation technique. Criteria for purity used to evaluate sampels revealed DNA preparations of high quality. Relationships of various Pichia species (hat-shaped spore formers and round spore formers), the imperfect counterparts, species fomrerly belonging to Endomucopsis, the bee-associated yeasts, and various Candida and Torulopsis species are being determined. DNA studies demonstrate that present taxonomic criteri used for species delimitaton areinadequate and require revision. From a practical aspect it is expected that the data from molecular comparisons will lead to the selection of firm criteria to be used for species differentiation. Such criteria will be chosen so even simply-equipped laboratories will be able to identify yeasts with ease and precision.